1. Field of the Invention
The present invention relates generally to a system and a method of control for a motor vehicle, and more particularly to a system and method for controlling torque provided to one or more wheels of a hybrid vehicle.
2. Background Art
Vehicle operators prefer that hybrid vehicles behave similarly to conventional motor vehicles, such as those having an internal combustion engine and an automatic transmission. One desirable characteristic is “hill holding” capability, or a lack of rollback when a vehicle is disposed on an inclined surface. In a conventional vehicle, the vehicle power train delivers enough torque at idle to the vehicle traction wheels to hold the vehicle on an inclined surface. In a hybrid vehicle, the engine may be turned off and therefore may not be available to provide torque. Moreover, starting or continuing operation of the engine to provide hill-holding torque wastes fuel and increases undesirable emissions when sufficient torque may be available from other sources. In addition, the use of an electrical machine in a hybrid vehicle to provide hill holding torque may be undesirable since it wastes battery power and increases cooling requirements.
Before applicants' invention there was a need for a system and a method that inhibits rollback of a hybrid vehicle disposed on an inclined surface. In addition, there was a need for a system and a method that could inhibit vehicle rollback on an inclined surface having a steeper grade than was accommodated by previous systems. In addition, there was a need for a system and method that improves operating efficiency of a hybrid vehicle during hill holding situations. In addition there was a need for a system and a method that could implement hill holding without relying on an electrical machine to provide torque, thereby improving component life and reducing cooling requirements. Problems associated with the prior art as noted above and other problems are addressed by applicants' invention as summarized below.